1. Field of the Invention
The present invention relates to a focal plane shutter for cameras having a shutter blade which can be actuated in both directions for opening as well as closing an opening of a photographing path by an electromagnetic actuating device, such as a motor.
2. Description of the Related Art
In a focal plane shutter for cameras, there are two types of focal plane shutter. One type is equipped with two shutter blades and another type has only one shutter blade. The former type having two shutter blades has been adopted to silver-salt film cameras as well as digital still cameras, while the latter having single shutter blade has been adopted only to digital still cameras. As an actuating source for operation of a shutter blade at the time of photographing, a coil spring has been generally used and it has been also proposed for many years that an electromagnetic device such as a motor is used.
It is known that when these shutter blades are respectively driven by actuating source of each electromagnetic device in case of a focal plane shutter equipped with two shutter blades as mentioned above, there is one type of focal plane shutter, wherein in reciprocating motion of two shutter blades forward motion is adopted for exposure and return motion is used for cocking.
On the other hand, in another type, both of the forward motion and the return motion in the reciprocating motions are used for exposure, but cocking is not executed. (i.e. Japanese Patent Application Publication: Toku Kai No.2001-188280). Furthermore, it is also known that in a focal plane shutter with only one shutter blade, such shutter blade is driven in the reciprocating motion using an electromagnetic actuating device as a source of driving. (refer to Japanese Patent Application Publication Toku Kai No.2001-188280)
It is required that a shutter blade should operate at high speed at the time of photographing. However, such shutter blade may be bounded or broken by the shock of the bound of a shutter blade when the shutter blade itself or the driving member operating the shutter blade is collided with a stopper for stopping the action of the shutter. When the bound is big, it may cause generation of exposure unevenness in case of a focal plane shutter with two shatter blades, while it may cause exposure unevenness and the smear phenomenon in case of a focal plane shutter with one shatter blade (for digital camera). Then, it is known that in case of a focal plane shutter using spring as an actuating source, a brake means is provided in order to brake action of the actuating member and the shutter blades at the termination stage of an exposure operation (refer to ,for example, Japanese Patent Application Publication: Toku Kai Hei No. 10-221744)
Providing the brake means mentioned above is also needed when a shutter blade is actuated by an electromagnetic device as an actuating source. It is because recently it has become possible to actuate a shutter blade at remarkably high speed, since the performance of a motor has been improved as well as weight reduction of a shutter blade has also progressed and so on. Accordingly, in a focal plane shutter with two shutter blades in which both of the forward motion and the return motion in the reciprocating motion are used for exposure, a brake means at the time of actuation in the forward motion and another brake means at the time of actuation of the return motion are necessary. Furthermore, recently, the demand for continuous photographing function is strong and it is required that time interval for starting the next photographing is shortened as much as possible. Therefore, a brake means at the time of the return motion and the brake means at the time of the forward motion are also needed when an exposure operation is performed only by the forward motion, while a cocking actuation is carried out by the return motion in order to carry out a cocking actuation at high speed and to acquire a stabilized cocking state quickly. Furthermore, in order to cope with the demand of shortening time interval necessary for starting the next photographing, the brake means at the time of the forward motion and the brake means at the time of return motion are needed, even in a shutter having only one shutter blade.
As mentioned above, in case that both brake means in the forward motion and in the return motion are needed, arranging each brake means individually causes increase of the number of parts. This is not only disadvantageous from viewpoint of cost but also very difficult to arrange a lot of parts in narrow space of the shutter base plate. Also, if each part is made small to make an arrangement in narrow space possible, an assembly work becomes difficult and a predetermined function cannot be actually obtained. Furthermore, it is necessary to device a certain proper means for the constitution as well as the layout of the brake means, the electromagnetic actuating device which occupies a relatively big space and the actuating member driving a shutter blade where the electromagnetic actuating device as an actuating source. Otherwise, the space occupied with such mechanism portion becomes large and consequently it causes large sizing of shutter unit.